Formulations, methods, and apparatus for remote triggering of frontally cured polymers

ABSTRACT

In some variations, the invention provides a curable adhesive formulation comprising a curable liquid precursor capable of frontal polymerization, wherein the liquid precursor comprises a monomer and a polymerization catalyst, and frontal-polymerization-triggering susceptors in contact with, or contained within, the liquid precursor. The susceptors may include conducting and/or magnetic solid particles capable of induction heating in the presence of a remotely applied electromagnetic field. Other variations provide a polymer-curing system comprising a curable liquid precursor, frontal-polymerization-triggering susceptors, and an apparatus configured to remotely produce an alternating electromagnetic field in line-of-sight with the susceptors (but not necessarily in line-of-sight with the liquid precursor), thereby generating induction heating to initiate the frontal polymerization. The susceptors may be about 0.1 wt % to about 50 wt % of the curable formulation. Other variations provide a method of curing an adhesive joint through an opaque barrier.

PRIORITY DATA

This patent application is a divisional application of U.S. patentapplication Ser. No. 14/556,090, filed on Nov. 28, 2014 (now allowed),which claims priority to U.S. Provisional Patent App. No. 61/910,297,filed on Nov. 30, 2013, each of which is hereby incorporated byreference herein.

FIELD OF THE INVENTION

The present invention generally relates to curable systems and curedpolymers suitable for adhesives and other applications.

BACKGROUND OF THE INVENTION

An adhesive is any substance applied to the surfaces of materials thatbinds them together and resists separation. Adhesives offer manyadvantages over binding techniques such as sewing, mechanical fastening,or thermal bonding. These include the ability to bind differentmaterials together, to distribute stress more efficiently across thejoint, increased design flexibility, and cost effectiveness.

Surface bonding techniques are preferred to mechanical fastenersprimarily due to their superior load-transfer characteristics. Forhigh-performance engineering applications, surface bonding is typicallyachieved using elevated temperature-cure, thermosetting adhesives. Thesethermosetting adhesives usually require temperatures of 120-200° C. for5-120 min to complete the bond. The most common ways of heating adhesivebonds are convection ovens, thermal blankets, and radiant heaters.

An ideal adhesive is one that has long pot life, but can be curedimmediately when needed. It is also highly desirable for the adhesive tocure without having to submit the entire part assembly to a large oven,thermal blanket, or radiant heater. Furthermore, for energy efficiency,one may also take advantage of the latent reaction heat from theadhesive system to sustain its own curing. Such an adhesive providestremendous advantage for flexible and efficient manufacturing processes.

Frontal polymerization offers attributes that meet the aboverequirements. Frontal polymerization is a localized reaction thatpropagates through the coupling of thermal diffusion and the Arrheniusdependence of an exothermic polymerization reaction. The result is alocalized thermal reaction zone that then propagates through thereactants as a thermal wave. Frontal polymerization exploits heatproduction because of exothermicity of the polymerization reactionitself and its dispersion by thermal conduction. If the amount ofdissipated heat is not too great, then a sufficient quantity of energyable to induce the polymerization of the monomer close to the hot zoneis provided. The result is the formation of a hot polymerization frontcapable of self-sustaining and propagating throughout the reactor.

Currently known frontal polymerization can only be triggered by direct(physical) contact to thermal (heat) sources or line-of-sight exposureto UV light. Yet, practical adhesives are often used to bond opticallyopaque parts for which neither triggered mechanisms is applicable.

Conventional approaches require either direct contact with a heat source(e.g., soldering iron) or line-of-sight exposure (UV light). Mostadhesive bonding processes occur in between two optically opaquesubstrates; thus the approaches in the known art are not applicable. Asolution to this problem is needed.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned needs in the art, aswill now be summarized and then further described in detail below.

In some variations, the invention provides a curable formulation (e.g.,an adhesive formulation) comprising:

(a) a curable liquid precursor capable of frontal polymerization,wherein the liquid precursor comprises a monomer and a polymerizationcatalyst; and

(b) one or more frontal-polymerization-triggering susceptors in contactwith, and/or contained within, the liquid precursor, wherein thesusceptors comprise conducting and/or magnetic solid particles capableof induction heating in the presence of a remotely appliedelectromagnetic field.

In some embodiments, the susceptors are present at a concentration fromabout 0.1 wt % to about 50 wt % in the curable formulation, such as aconcentration from about 5 wt % to about 25 wt % in the curableformulation.

The monomer may be an epoxy resin, for example. The polymerizationcatalyst may be a latent catalyst, such as a latent catalyst comprisinga tertiary amine (e.g., an imidazole compound) and/or a borontrifluoride-amine complex. In some embodiments, the liquid precursorfurther comprises an accelerator, such as a polyol.

In some embodiments, the formulation comprises a single susceptor regionor layer adjacently disposed in contact with the liquid precursor. Inthese or other embodiments, the formulation comprises multiplesusceptors dissolved and/or suspended within the liquid precursor.

The conducting and/or magnetic solid particles may contain a materialselected from the group consisting of iron, nickel, zinc, chromium,oxides or alloys containing iron, oxides or alloys containing nickel,oxides or alloys containing zinc, oxides or alloys containing chromium,carbon, and combinations thereof.

Other variations provide a polymer-curing system comprising:

(a) a curable liquid precursor capable of frontal polymerization,wherein the liquid precursor comprises a monomer and a polymerizationcatalyst; and

(b) one or more frontal-polymerization-triggering susceptors in contactwith, and/or contained within, the liquid precursor, wherein thesusceptors comprise conducting and/or magnetic solid particles capableof induction heating in the presence of an electromagnetic field; and

(c) an apparatus configured to remotely produce an alternatingelectromagnetic field in line-of-sight with the susceptors, therebygenerating the induction heating to initiate the frontal polymerization.

In some embodiments, the alternating electromagnetic field is not inline-of-sight with at least a portion of the curable liquid precursor.In certain embodiments, the alternating electromagnetic field is not inline-of-sight with any of the curable liquid precursor.

The susceptors may be present at a concentration from about 0.1 wt % toabout 50 wt % in the liquid precursor. The monomer may be, but is notlimited to, an epoxy resin. The polymerization catalyst may be a latentcatalyst, such as a latent catalyst comprising a tertiary amine (e.g.,an imidazole compound) and/or a boron trifluoride-amine complex. In someembodiments, the liquid precursor further comprises an accelerator, suchas a polyol.

The system may include a single susceptor region or layer adjacentlydisposed in contact with the liquid precursor. Alternatively, oradditionally, the system may include multiple susceptors dissolvedand/or suspended within the liquid precursor.

In some embodiments of the system, the conducting and/or magnetic solidparticles contain a material selected from the group consisting of iron,nickel, zinc, chromium, oxides or alloys containing iron, oxides oralloys containing nickel, oxides or alloys containing zinc, oxides oralloys containing chromium, carbon, and combinations thereof.

Other variations provide a method of curing an adhesive joint, themethod comprising:

(a) selecting a curable liquid precursor capable of frontalpolymerization, wherein the liquid precursor comprises a monomer and apolymerization catalyst;

(b) selecting frontal-polymerization-triggering susceptors to initiatepolymerization upon remote command, wherein the susceptors compriseconducting and/or magnetic solid particles capable of induction heatingin the presence of an electromagnetic field;

(c) combining the curable liquid precursor and the susceptors, togenerate a mixture;

(d) transferring the mixture to an adhesive joint; and

(e) applying an electromagnetic signal to remotely initiatepolymerization of the precursor,

wherein the electromagnetic signal, due to an opaque barrier or forother reasons, is not in line-of-sight with at least a portion of thecurable liquid precursor.

In some methods, the electromagnetic signal is not in line-of-sight withany of the curable liquid precursor. Note, however, that this is not arequirement for the method to work. Rather, it represents a practicalconfiguration of remote curing, despite an opaque barrier (if present),that is enabled by the susceptors.

The susceptors are present at a concentration from about 0.1 wt % toabout 50 wt % in the mixture, in some embodiments. The mixture mayinclude a single susceptor region or layer adjacently disposed incontact with the liquid precursor. Alternatively, or additionally, themixture may include multiple susceptors dissolved and/or suspendedwithin the liquid precursor.

The electromagnetic signal is preferably an alternating electromagneticfield. An alternating electromagnetic field (such as, but not limitedto, an RF energy field) may be generated to supply the electromagneticsignal to remotely initiate polymerization of the precursor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic of direct thermal-contact triggered frontalpolymerization, the principles of which may be applied in someembodiments.

FIG. 2 depicts a schematic of remotely blind-triggered frontalpolymerization, according to some embodiments of the invention.

FIG. 3 depicts a schematic of remotely blind-triggered frontalpolymerization, according to some embodiments of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The compositions, formulations, methods, apparatus, and systems of thepresent invention will be described in detail by reference to variousnon-limiting embodiments.

This description will enable one skilled in the art to make and use theinvention, and it describes several embodiments, adaptations,variations, alternatives, and uses of the invention. These and otherembodiments, features, and advantages of the present invention willbecome more apparent to those skilled in the art when taken withreference to the following detailed description of the invention inconjunction with the accompanying drawings.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contextclearly indicates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as is commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs.

Unless otherwise indicated, all numbers expressing conditions,concentrations, dimensions, and so forth used in the specification andclaims are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending at least upona specific analytical technique.

The term “comprising,” which is synonymous with “including,”“containing,” or “characterized by” is inclusive or open-ended and doesnot exclude additional, unrecited elements or method steps. “Comprising”is a term of art used in claim language which means that the named claimelements are essential, but other claim elements may be added and stillform a construct within the scope of the claim.

As used herein, the phrase “consisting of” excludes any element, step,or ingredient not specified in the claim. When the phrase “consists of”(or variations thereof) appears in a clause of the body of a claim,rather than immediately following the preamble, it limits only theelement set forth in that clause; other elements are not excluded fromthe claim as a whole. As used herein, the phrase “consisting essentiallyof” limits the scope of a claim to the specified elements or methodsteps, plus those that do not materially affect the basis and novelcharacteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consistingessentially of,” where one of these three terms is used herein, thepresently disclosed and claimed subject matter may include the use ofeither of the other two terms. Thus in some embodiments not otherwiseexplicitly recited, any instance of “comprising” may be replaced by“consisting of” or, alternatively, by “consisting essentially of.”

This invention is premised, at least in part, on the discovery of asolution to the problem (in conventional approaches) that frontalpolymerization is triggered by direct physical contact to heatingsources or by line-of-sight exposure to light. Variations of thisinvention utilize remote and blind triggering mechanisms, preferablybased on induction heating, to initiate frontal (also known as frontallycured) polymerization. These variations allow adhesives to bondoptically opaque parts, for which conventional approaches are inferioror do not work at all.

Frontal polymerization systems are those that when subjected to atrigger, the curing of the formulation will start locally. The triggermay then be removed. The curing front will propagate outward by virtueof the initial reaction heat. Frontal polymerization systems known inthe art can be triggered by direct contact with heating sources (e.g. asoldering iron). UV light-triggered frontal curing is also known. Thelatter can be applied in a remote fashion, but line-of-sight exposure tothe monomer is required.

This invention, in some variations, discloses formulations(compositions) and processes to utilize frontal polymerization systemstriggered both remotely and without need for line-of-sight with theformulations. Specific embodiments include triggering mechanisms such asinduction heating to initiate the frontal curing of polymers, whereinthe curing is initiated locally from the induction heating of magneticand/or conducting particles. When combined with a resin system capableof frontal polymerization, the initial inductive heating trigger maylead to complete curing of the system.

Such a system, which can be (but is not limited to) an adhesive,requires a sufficiently long pot life to be easily workable within thetime frame of common industrial manufacturing processes. Preferred resinformulas provide room-temperature pot lives extending to multiple hoursin order to provide maximum processing convenience by combining extendedworkability and rapid cure time.

In some variations, the invention provides a curable formulation (e.g.,an adhesive formulation) comprising:

(a) a curable liquid precursor capable of frontal polymerization,wherein the liquid precursor comprises a monomer and a polymerizationcatalyst; and

(b) one or more frontal-polymerization-triggering susceptors in contactwith, and/or contained within, the liquid precursor, wherein thesusceptors comprise conducting and/or magnetic solid particles capableof induction heating in the presence of a remotely appliedelectromagnetic field.

In preferred embodiments, the polymerization is step-growthpolymerization. Step-growth polymerization refers to a type ofpolymerization mechanism in which bi-functional or multi-functionalmonomers react to form first dimers, then trimers, then longeroligomers, and eventually long-chain polymers.

In some embodiments, the polymerization is chain-growth polymerization.Chain-growth polymerization is a polymerization technique whereunsaturated monomer molecules add onto the active site of a growingpolymer chain one at a time. Chain-growth polymerization includesfree-radical polymerization, cationic polymerization, anionicpolymerization, and coordination polymerization.

When polymerization is step-growth polymerization, the monomer may be anepoxy resin, for example. Epoxy resins, also known as polyepoxides, area class of reactive prepolymers and polymers which contain epoxidegroups. Epoxy resins may be reacted (cross-linked) either withthemselves through catalytic homopolymerization, or with a wide range ofco-reactants including polyfunctional amines, acids (and acidanhydrides), phenols, alcohols, and thiols. These co-reactants are oftenreferred to as hardeners or curatives, and the cross-linking reaction iscommonly referred to as curing.

Polymerization may be catalyzed with a latent catalyst contained withinthe liquid precursor. A “latent catalyst” is a substance which isrelatively stable, but which becomes activated at a curing temperatureto produce a substance having acidic or basic properties. Suitablelatent catalysts ought to be readily miscible with the epoxy resin orother monomers. The mixtures ought to remain stable for as long aspossible at room temperature under standard storage conditions. Thetemperatures required for curing are preferably not excessively high,such as below 200° C. Lower curing temperatures allow energy costs to besaved and unwanted secondary reactions avoided.

Latent catalysts available commercially include, for example, adducts ofboron trifluoride with amines (e.g., BF₃-monoethylamine), quaternaryphosphonium compounds, and dicyandiamide. In some embodiments, a latentcatalyst comprises a tertiary amine (e.g., an imidazole compound). Inthese or other embodiments, a latent catalyst comprises a borontrifluoride-amine complex.

The liquid precursor may further comprise an accelerator to speed up thereaction kinetics in the curing process. In some embodiments, theaccelerator is or includes a polyol. The polyol may be, for example, aC₂-C₂₄ polyol, such as ethylene glycol, 1,2-propanediol, 1,4-butanediol,2,3-butanediol, or a derivative thereof. The accelerator may act as aco-catalyst (e.g., to reduce the activation energy of curing), or mayact to enhance the catalytic activity of the polymerization catalyst.

In some embodiments, the susceptors are present at a concentration fromabout 0.1 wt % to about 50 wt % in the curable formulation, such as aconcentration from about 5 wt % to about 25 wt % in the curableformulation. In certain embodiments, the susceptors are present at aconcentration of about 0.2, 0.5, 1.0, 1.5, 2, 3, 4, 5, 7.5, 10, 15, 20,25, 30, 35, 40, 45, or 50 wt %, including any intermediate rangesbetween these recited values (such as 0.5-5 wt % or 2-30 wt %).

In some embodiments, the formulation comprises a single susceptor regionor layer adjacently disposed in contact with the liquid precursor (asdepicted in FIG. 2). In these or other embodiments, the formulationcomprises multiple susceptors dissolved and/or suspended within theliquid precursor (as depicted in FIG. 3).

The conducting and/or magnetic solid particles may contain a materialselected from the group consisting of iron, nickel, zinc, chromium,oxides or alloys containing iron, oxides or alloys containing nickel,oxides or alloys containing zinc, oxides or alloys containing chromium,carbon, and combinations thereof.

Susceptor materials may also be characterized by magnetic permeability.“Permeability” μ is a measure of the ability of a material to supportthe formation of a magnetic field within itself. “Relative permeability”is the ratio of the permeability μ of a specific material to thepermeability μ of free space. In some embodiments, selected susceptormaterials are characterized by a relative permeability μ/μ₀ of about 100or higher, such as about 500, 1000, or higher.

Other variations provide a polymer-curing system comprising:

(a) a curable liquid precursor capable of frontal polymerization,wherein the liquid precursor comprises a monomer and a polymerizationcatalyst; and

(b) one or more frontal-polymerization-triggering susceptors in contactwith, and/or contained within, the liquid precursor, wherein thesusceptors comprise conducting and/or magnetic solid particles capableof induction heating in the presence of an electromagnetic field orelectromagnetic field; and

(c) an apparatus configured to remotely produce an alternatingelectromagnetic field or signal in line-of-sight with at least some ofthe susceptors, thereby generating induction heating to initiate thefrontal polymerization.

“Line-of-sight” between a source and an object means that anelectromagnetic signal (photons) is able to travel from the source tothe object without quantum tunneling. In some embodiments, thealternating electromagnetic field is in line-of-sight with essentiallyall of the susceptors present. In these or other embodiments, thealternating electromagnetic field is not in line-of-sight with at leasta portion of the curable liquid precursor. In certain embodiments, thealternating electromagnetic field is not in line-of-sight with any ofthe curable liquid precursor. The electromagnetic field may be out ofthe line-of-sight with the liquid precursor due to the presence of anopaque barrier, a lack of alignment between electromagnetic field sourceand liquid precursor region, a source or cause of interference, or otherreasons. These embodiments are in contrast to UV ignition, whichrequires line-of-sight exposure with the curable liquid precursor forcuring.

Induction heating works by exposing a conductive or magnetic material toan electromagnetic field. Any material that heats up when exposed to anelectromagnetic field is called a “susceptor” material. Theelectromagnetic field can induce heating through two mechanisms. If thesusceptor material is conductive, eddy currents are induced in theconductor, and the conductor will heat due to resistive effects. If thematerial is magnetic, hysteresis losses from themagnetization-demagnetization process cause heating. This mechanism ofheating is called hysteresis heating.

The physics of electromagnetic induction applies to both Joule heating(eddy current losses) and magnetic heating (hysteresis losses). Eithermechanism may dominate, or both may be important. In non-conductive, butferromagnetic materials (e.g., ceramic ferrites), induction heatingarises from only magnetic hysteresis, while in conductive, butnon-magnetic materials (e.g., aluminum flakes), induction heating arisesfrom eddy currents (Joule heating) while remaining unaffected by themagnetic field. In the case of iron-based ferromagnetic materials, bothmagnetic hysteresis and eddy current heating contribute significantly toheating. In some embodiments, once local curing starts, theelectromagnetic field is turned off and the activated susceptor returnsto a non-activated state. In other embodiments, the electromagneticfield remains on for some period of time (optionally, until curing iscomplete).

The electromagnetic field can be viewed as the combination of anelectric field and a magnetic field. The electric field is produced bystationary charges, and the magnetic field is produced by moving charges(currents).

The electromagnetic field or signal may be in, or derived from, analternating electromagnetic field. That is, an alternatingelectromagnetic field may be generated to supply the electromagneticsignal to remotely initiate polymerization of the precursor. Analternating electromagnetic field results from alternating currentmoving through a conductor. In other embodiments, the electromagneticsignal is in, or derived from, a non-alternating electromagnetic field.A non-alternating electromagnetic field results from direct currentmoving through a conductor.

The electromagnetic field or signal may have a frequency from about 3kHz to 300 GHz, i.e. radio frequency. In some embodiments, theelectromagnetic field or signal has a frequency from about 50 kHz toabout 100 MHz, such as about 0.1 MHz to about 10 MHz. In variousembodiments, the electromagnetic field or signal has a frequency ofabout 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 MHz. Someembodiments employ high-frequency (10 MHz or higher) electromagneticsignals.

The electromagnetic field may be produced using any known apparatuscapable of generating an electromagnetic field. For example, aninduction unit operates by sending an alternating current through aconductive coil, which then generates an alternating electromagneticfield. The magnitude of the electromagnetic field may be adjusted byincreasing the amount of current that enters the coil and thusincreasing the electromagnetic field amplitude. For example, aninduction unit that is rated between 0 and 3000 W of load power may beadjusted to 1%, 5%, 10%, 20%, or 50% power (as % of maximum power), toadjust the electromagnetic field.

The susceptors may be present at a concentration from about 0.1 wt % toabout 50 wt % in the liquid precursor. In certain embodiments, thesusceptors are present at a concentration of about 0.2, 0.5, 1.0, 1.5,2, 3, 4, 5, 7.5, 10, 15, 20, 25, 30, 35, 40, 45, or 50 wt %, includingany intermediate ranges between these recited values (such as 0.5-5 wt %or 2-30 wt %).

The system may include a single susceptor region or layer adjacentlydisposed in contact with the liquid precursor (see FIG. 2 for anillustration, and Example 1 for details of FIG. 2). Following curing, aportion or all of the susceptor region or layer may be removed, since ittypically will not mix with the liquid precursor. Alternatively, thesystem may include multiple susceptors (e.g., susceptor particles)dissolved and/or suspended within the liquid precursor (see FIG. 3 foran illustration, and Example 2 for details of FIG. 3). In someembodiments, the system includes a susceptor region or layer adjacentlydisposed in contact with the liquid precursor as well as multiplesusceptors (e.g., susceptor particles) dissolved and/or suspended withinthe liquid precursor.

In some embodiments of the system, the conducting and/or magnetic solidparticles contain a material selected from the group consisting of iron,nickel, zinc, chromium, oxides or alloys containing iron, oxides oralloys containing nickel, oxides or alloys containing zinc, oxides oralloys containing chromium, carbon, and combinations thereof. In someembodiments, selected susceptor materials are characterized by arelative permeability μ/μ₀ of about 100 or higher, such as about 500,1000, or higher.

In some embodiments, the susceptors comprise magnetic powder with Curietemperature-limited heating. These materials use hysteresis heating and,if all other heating effects are dominated by the hysteresis heating,will not heat beyond their Curie temperature (the temperature at which amaterial's permanent magnetism changes to induced magnetism). Magneticpowders (e.g., ferrite-based powders) may be chosen whose Curietemperature can be matched to the desired processing temperature of theadhesive system.

The monomer may be, but is not limited to, an epoxy resin. In someembodiments, the monomer is not a vinyl monomer. The polymerizationcatalyst may be a latent catalyst, such as a latent catalyst comprisinga tertiary amine (e.g., an imidazole compound) and/or a borontrifluoride-amine complex. In some embodiments, the liquid precursorfurther comprises an accelerator, such as a polyol.

Some embodiments provide a method of curing an adhesive joint, themethod comprising:

(a) selecting a curable liquid precursor capable of frontalpolymerization, wherein the liquid precursor comprises a monomer and apolymerization catalyst;

(b) selecting frontal-polymerization-triggering susceptors to initiatepolymerization upon remote command, wherein the susceptors compriseconducting and/or magnetic solid particles capable of induction heatingin the presence of an electromagnetic field;

(c) combining the curable liquid precursor and the susceptors, togenerate a mixture;

(d) transferring the mixture to an adhesive joint; and

(e) applying an electromagnetic signal to remotely initiatepolymerization of the precursor,

wherein the electromagnetic signal, due to an opaque barrier, is not inline-of-sight with at least a portion of the curable liquid precursor.

In some methods, the electromagnetic signal is not in line-of-sight withany of the curable liquid precursor. Note, however, that this is not arequirement for the method to work. Rather, it represents a practicalconfiguration of remote curing, despite an opaque barrier (if present),that is enabled by the susceptors and electromagnetic signal.

Induction heating is the process of heating an electrically conductingobject (usually a metal) by electromagnetic induction, where eddycurrents are generated within the metal and resistance leads to Jouleheating of the metal. An induction heater includes an electromagnet,through which a high-frequency alternating current is passed. Heat mayalso be generated by magnetic hysteresis losses in materials.

The susceptors are present at a concentration from about 0.1 wt % toabout 50 wt % in the mixture, in some embodiments. The mixture mayinclude a single susceptor region or layer adjacently disposed incontact with the liquid precursor. Alternatively, or additionally, themixture may include multiple susceptors dissolved and/or suspendedwithin the liquid precursor.

In steps (c)-(e), the mixture may be characterized by a room-temperature“pot life” which allows sufficient time before applying, to the mixture,an electromagnetic signal to cause heating for remotely initiatingpolymerization. In some embodiments, the pot life is from about 0.1 hrto about 24 hr, such as about 0.5 hr to about 12 hr, or about 1 hr toabout 9 hr.

Some embodiments provide a method of curing a polymer, the methodcomprising:

(a) selecting a curable liquid precursor capable of frontalpolymerization, wherein the liquid precursor comprises a monomer and apolymerization catalyst;

(b) selecting frontal-polymerization-triggering susceptors to initiatepolymerization upon remote command, wherein the susceptors compriseconducting and/or magnetic solid particles capable of induction heatingin the presence of an electromagnetic field;

(c) combining the curable liquid precursor and the susceptors, togenerate a mixture; and

(d) applying an electromagnetic signal to remotely initiatepolymerization of the precursor,

wherein the electromagnetic signal, due to an opaque barrier, is not inline-of-sight with at least a portion of the curable liquid precursor.

EXAMPLES

Various embodiments of the invention will now be further described byreference to the Examples and accompanying drawings (FIGS. 1-3), whichare non-limiting examples and drawings for illustration purposes.

Example 1: Remotely Blind-Triggered Frontal Polymerization

An illustration of soldering iron-induced frontal polymerization isshown in FIG. 1. In step I (schematic 101), a hot soldering iron 120 isimmersed into an uncured liquid 110 contained in a tube. Alternatively,the soldering iron 120 may contact the outside wall of the tube. Thisdirect contact to a heat source 120 induces a local hot spot at thecontact point 125. When sufficient heat is built up locally, curing willstart, as depicted in step II (schematic 102). Once the localized curingstarts, the soldering iron 120 can be removed in step III (schematic103). Due to the exothermic nature of the curing, the local fast curingproduces sufficient heat that induces curing in its proximity 130, whichfurther produces heat due to the release of the latent reaction energy.This process results in the self-propagation of the reaction front (ofcured polymer 130) as shown in step IV (schematic 104), leadingeventually to the curing of the entire system 130 as shown in step V(schematic 105). Alternatively, the initial trigger can be a UV source,when the system is UV curable. Again, when the curing reaction istriggered locally by UV light, the light can be removed, yet the curingcan self-propagate via heat released from the curing reaction.

Bisphenol A diglycidyl ether (EPON 825) is obtained from Momentive,polyethylene glycol diol (diol, molecular weight of 200) and1,2-dimethylimidazole (DMI) are purchased from Aldrich. All regents areused as received.

The exact formulations of various curing mixtures are provided inTable 1. In a typical experiment, weighed epoxy resin EPON 825 (maincomponent), PEO-diol (accelerator), and dimethylimidazole (catalyst) aremixed in a 30 mL glass vial. The pot life of each formulation in Table 1is evaluated by leaving the mixture under ambient conditions (i.e.room-temperature aging). None of the three formulations shows visiblechange in flowability before and after aging for 8 hours, signifying apot life longer than 8 hours.

To evaluate the curing speed of each formulation, two drops of theliquid mixture are added into an aluminum dish using a pipette. Thealuminum dish is immediately put into an oven preset at a certaintemperature. Upon gelation, the curing mixture suddenly becomes dark,accompanied by solidification. The sudden change in darkness is used asthe indicator of cure time. As such, the curing time of the differentformulations are provided in Table 1.

TABLE 1 Fast cure epoxy adhesive formulations. Cure time Cure time No.EPON DMI Diol 130° C. 150° C. 1 10 g 0.6 g 1 g 75 s 45 s 2 10 g 0.6 g 2g 60 s 45 s 3 10 g 0.6 g 0 g 180 s  90 s

The data in Table 1 show that addition of the diol significantlyaccelerates the curing, while maintaining pot life longer than 8 hours.For the same formulation, curing at 150° C. is also noticeably fasterthan curing at 130° C. Both formulations 1 and 2 in Table 1 cured around45 seconds at 150° C.

Direct Thermal Contact Initiated Frontal Curing

Following the scheme shown in FIG. 1, the directthermal-contact-initiated frontal polymerization of formulation 1(Table 1) 110 is evaluated by immersing the tip of a soldering iron 120(at a temperature of about 260° C.) into the vial that contains theliquid 110. After about 1 minute of the direct thermal contact, the toplayer of the liquid becomes significantly darkened, signifying the startof the frontal polymerization (producing polymer 130). At this point,the soldering iron is removed completely from the vial. The curing front130, as indicated by the darkening line, is observed to propagate on itsown until completion. The frontal propagation speed for this formulationis roughly 0.5 cm/min.

Remote Induction-Induced Thermal Curing

About 5 vol % of iron particles are dispersed in the epoxy adhesive(epoxy resin EPON 825). A small drop of the resulting mixture is put inbetween two glass slides. A high-frequency magnetic field (Power Cube32/900 Generator, 2.8 kW, 750-1150 kHz, manufactured by Ceia) placedclose to the glass slides (no direct contact) is turned on. Within 20seconds, the adhesive becomes very dark, indicating its curing. Thefield is then turned off. Upon completion of this process, the glassslides are found to adhere to each other well.

Remote Induction Induced Frontal Curing

In this Example 1, induction heating-induced fast curing and directheating-induced frontal curing have been demonstrated separately. Bycombining these two approaches, induction heating-induced (a remote andblind actuation) frontal curing may be carried out.

Some embodiments of a process of remote induction-induced frontal curingare illustrated in FIG. 2. An induction susceptor 220 may be placed incontact with the top of a curing liquid 210 in a glass vial, in step I(schematic 201). A remote non-contact induction unit that generates anelectromagnetic field 230 is turned on, which activates the susceptor225 and triggers local curing of polymer 240 in step II (schematic 202).Once the local curing starts, the electromagnetic field 230 may beturned off so that the activated susceptor 225 returns to itsnon-activated form 220 in step III (schematic 203). The frontal curingmechanism allows the curing to self-propagate in step IV (schematic 204)until the curing comes to completion in step V (schematic 205) toproduce a cured polymer 240.

Example 2: Remotely Blind-Triggered Frontal Polymerization

Whereas the susceptor in Example 1, FIG. 2 is a bulk material, thesusceptor may also be in the form of particles suspended within theadhesive. This alternative process is illustrated in FIG. 3.

A plurality of induction susceptor particles 320 are dispersed within acuring liquid 310 in a glass vial, in step I (schematic 301). A remotenon-contact induction unit that generates an electromagnetic field 330is turned on, which activates the susceptors 325 and triggers localcuring of polymer 340 in step II (schematic 302). Once the local curingstarts, the electromagnetic field 330 is turned off and the activatedsusceptors 325 return to non-activated form 320 in step III (schematic303). The frontal curing mechanism allows the curing to self-propagatein step IV (schematic 304) until the curing comes to completion in stepV (schematic 305) to produce a cured polymer 340.

The operating steps in FIG. 3 are similar to the steps shown in FIG. 2.However, due to the dispersion of the induction susceptor particleswithin the adhesive, the initial curing also occurs within the vicinity340 around the multiple particles 320. This triggers multiple reactionfronts 340 (shown in FIG. 3 as three distinct reaction zones forillustration purposes only) within the adhesive. The multiple reactionfronts 340 within the curing mixture allows complete curing within ashorter period of time compared to the process outlined in FIG. 2.

Example 3: Remotely Blind-Triggered Frontal Polymerization

The effectiveness of the methods described in Examples 1 and 2, and inFIGS. 2 and 3, is validated by comparing three resins with similarcuring mechanisms that display slightly different frontal cureproperties (Table 2). Resin 1 is composed of a common epoxy resin (Epon828, Momentive) and a curing catalyst (B-110, Leepoxy Plastics). Epon828 is bisphenol A diglycidyl ether. B-110 is a BF₃-amine adduct. Resin2 is modeled after Resin 1 but is doped with an epoxy that cures with agreater exotherm than Epon 828, trimethylolpropane triglycidyl ether(TMPTGE, Sigma). Resin 3 is a version of Resin 1 diluted with 10%poly(ethylene glycol) (PEG, Sigma). Each resin is tested for its frontalcuring properties for fronts proceeding downward or upward as in FIG. 2as well as outward away from a point source as in FIG. 3.

TABLE 2 Frontal polymerization speeds for Resins 1, 2, and 3. FrontalGeometry Downward Downward Downward Upward Spherical Large* Medium*Small* Large* Large* Resin (mm/min) (mm/min) (mm/min) (mm/min) (mm/min)Resin 1 24.0 20.3 11.2 7.8 Complete** Resin 2 17.1 15.5 10.5 7.1Incomplete Resin 3 15.0 14.7 Incomplete Incomplete Incomplete *Large =2.5 cm vials; Medium = 1 cm vials; Small = 0.5 cm vials (all vialdiameters). **Not calculated; front extended radially at differentspeeds in different directions.

TABLE 3 Bulk curing properties of Resins 1, 2, and 3 loaded with 15 wt %Fe₃O₄, when subjected to varying RF (1 MHz) exposure times. Shore DShore D Shore D Time Exposed to RF Hardness, Hardness, Hardness,(seconds) Resin 1 Resin 2 Resin 3 20 — — — 25 61 — — 30 81 51 — 35 80 69— 40 78 70 64 45 78 70 55 55 77 — 60

The same resins are then compared for their curing rates when cured viainductive heating of susceptor particles. Resins 1, 2, and 3 are filledwith 15 wt % Fe₃O₄, and 150 mg of resin is added to a small glass vialand exposed to −1 MHz RF energy at 10% power. The resulting Shore Dhardness values are shown in Table 3 below.

Comparing Tables 2 and 3, it is discernable that Resin 1 is able tofrontally propagate upward and radially, in addition to the traditionaldownward direction. Resin 1 is tuned to propagate away from thesusceptor particles. Tuning a resin system for spherical frontalpropagation involves a complex analysis of many variables includingreaction rate and exothermicity, heat capacity, thermal conductivity,and viscosity. However, the correlation between the bulk sphericalfrontal propagation effect and the micron-scale curing away fromsusceptor particles provides, to a skilled artisan, an accessible meansof optimizing a resin's effectiveness of curing these systems viaRF-activated susceptors.

In this detailed description, reference has been made to multipleembodiments and to the accompanying drawings in which are shown by wayof illustration specific exemplary embodiments of the invention. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatmodifications to the various disclosed embodiments may be made by askilled artisan.

Where methods and steps described above indicate certain eventsoccurring in certain order, those of ordinary skill in the art willrecognize that the ordering of certain steps may be modified and thatsuch modifications are in accordance with the variations of theinvention. Additionally, certain steps may be performed concurrently ina parallel process when possible, as well as performed sequentially.

All publications, patents, and patent applications cited in thisspecification are herein incorporated by reference in their entirety asif each publication, patent, or patent application were specifically andindividually put forth herein.

The embodiments, variations, and figures described above should providean indication of the utility and versatility of the present invention.Other embodiments that do not provide all of the features and advantagesset forth herein may also be utilized, without departing from the spiritand scope of the present invention. Such modifications and variationsare considered to be within the scope of the invention defined by theclaims.

What is claimed is:
 1. A method of curing an adhesive joint, said methodcomprising: (a) selecting a curable liquid precursor capable of frontalpolymerization, wherein said liquid precursor comprises a monomer and apolymerization catalyst; (b) selecting frontal-polymerization-triggeringsusceptors to initiate polymerization upon remote command, wherein saidsusceptors comprise conducting and/or magnetic solid particles capableof induction heating in the presence of an electromagnetic field; (c)combining said curable liquid precursor and said susceptors, to generatea mixture; (d) transferring said mixture to an adhesive joint; and (e)applying an electromagnetic signal to remotely initiate polymerizationof said precursor, wherein said electromagnetic signal, due to an opaquebarrier, is not in line-of-sight with at least a portion of said curableliquid precursor.
 2. The method of claim 1, wherein said electromagneticsignal is not in line-of-sight with any of said curable liquidprecursor.
 3. The method of claim 1, wherein said susceptors are presentat a concentration from about 0.1 wt % to about 50 wt % in said mixture.4. The method of claim 1, wherein said mixture comprises a singlesusceptor region or layer adjacently disposed in contact with saidliquid precursor.
 5. The method of claim 1, wherein said mixturecomprises multiple susceptors dissolved and/or suspended within saidliquid precursor.